As polyester resins that are obtained by polycondensation of a polycarboxylic acid and a polyhydric alcohol, for example, polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate and polybutylene naphthalate are known. Particularly, polyethylene terephthalate is a resin that has excellent transparency, heat resistance, chemical resistance, hygienic properties, staining properties, economic efficiency and the like; therefore, it is widely used in, for example, fibers; film applications; beverage containers (PET bottles) of carbonated drinks, juice drinks, mineral waters and the like; containers of cosmetic and medical products; detergent and shampoo containers; electrophotographic toners; and packaging materials such as food packages, pharmaceutical packages and wrapping materials.
However, despite being crystalline resins, polyester resins, particularly polyethylene terephthalate, have an extremely slow crystallization rate; therefore, the range of molding conditions applicable thereto is very narrow and it is thus difficult to improve the processing cycle, limiting the use of polyester resins as molding materials. In addition, since molded articles obtained by molding polyester resins have low thermal deformation temperatures, there is a problem that their serviceable temperatures are limited.
As a method of improving the crystallization rate of a polyester resin, for example, a method of adding a nucleating agent is employed. Examples of the nucleating agent include polymers, minerals, metal salts of organic and inorganic acids, glass powders and metal powders, more specifically, olefins such as low-density polyethylenes, high-density polyethylenes and linear low-density polyethylenes; minerals (clays) such as graphite, talc and kaolin; metal oxides such as zinc oxide, alumina and magnesium oxide; silica compounds such as silica, calcium silicate and magnesium silicate; metal carbonates such as magnesium carbonate, calcium carbonate, sodium carbonate and potassium carbonate; barium sulfate; calcium sulfate; sodium benzoate; aluminum p-tert-butyl benzoate; metal aromatic phosphates; dibenzylidene sorbitol; sulfonamide compounds; and sulfonimide compounds. For example, in Patent Documents 1 to 4, the present inventions have proposed to incorporate a metal salt of a sulfonamide compound or a metal salt of a sulfonimide compound as a nucleating agent of a polyester resin.
As a method of incorporating a nucleating agent into a polyester resin, for example, a method in which a polyester resin and a nucleating agent are blended in advance using a Henschel mixer, a tumbler mixer or the like and the resulting mixture is fed to an extruder using a feeder or a method in which a polyester resin and a nucleating agent are separately fed to an extruder using quantitative feeders is employed. In these methods, since the nucleating agent and the polyester resin are in powder form and thus blown up into the air during operation, there is a problem that the working environment is adversely affected and contaminated. Accordingly, in recent years, the use of a method of adding a nucleating agent-containing masterbatch is becoming the main trend.
However, when a large amount of a metal salt compound is incorporated into a polyester resin to produce a masterbatch, there is a problem that the resultant cannot be granulated due to a marked reduction in the molecular weight of the polyester resin or the resulting masterbatch is colored and this leads to deterioration of the outer appearance of a molded article containing the masterbatch.
As a method of producing a masterbatch, for example, Patent Document 5 proposes a production method which comprises a first step of kneading a polyester resin and a metal salt compound and a second step of further adding and kneading the polyester resin with a kneaded product obtained in the first step.